H2 oxidation on doped yttrium chromites/yttrium stabilized zirconia anode of solid oxide fuel cell
Introduction
Ni/YSZ cermet is the most commonly used anode in SOFC with H2 fed as fuel, but it would degrade in hydrocarbon and impurity containing syngas due to coking and poisoning by contaminants such as S [1]. Therefore, alternative materials are long desired to overcome these drawbacks of Ni/YSZ anode. Perovskite oxides such as doped lanthanum chromites [2], [3], [4], [5] and lanthanum doped strontium titanate, etc. have been investigated extensively as potential anode materials [6], [7], [8], [9]. The ABO3 formula allows not only wide size variety at A and B sites but also different valence combination, making the electrical and catalytic properties adjustable [10]. YCrO3 perovskite has been examined as possible interconnect material as they display advantages over the traditional LaCrO3 based materials in terms of the chemical expansion and compatibility with yttrium stabled zirconia (YSZ) electrolyte [11], [12], [13], [14], [15], [16], [17]. In addition of working as interconnect, YCrO3 based materials have been evaluated as potential anode after multiple doping at A and B sites: an anode made from Ca and Co doped YCrO3 lately has demonstrated an encouraging performance in H2, and more importantly, displayed good tolerance towards 20 ppm H2S [18]. However to our best knowledge, besides the electrode performance research, no investigation regarding the anode reaction mechanism on YCrO3 based materials has been reported. Understanding on electrode reaction mechanism is important for the tailoring of material to further optimize electrode performance. In fact, on Pt and Ni-based anodes, mechanism and kinetics studies have been extensively investigated [19], [20], [21], [22], [23], [24], [25], [26], [27]. In these publications, it is generally recognized that most electrochemical reaction takes place at three-phase boundary (3PB) area and that the electrode performance is heavily dependent on the microstructure and electrode composition [25], [28]. It is also known from the EIS studies that the charge transfer process is relatively fast as compared to the adsorption and diffusion processes [29], [30], [31]. On the other hand, due to the variation on raw materials, electrode composition, manufacturing conditions and etc., different conclusions concerning reaction path and rate-limiting steps are also drawn out by different groups. These existent observations from Ni-based anodes could be relevant to the study of YCrO3 based anodes as well.
In this work, in order to gain knowledge of anode reaction on YCrO3 materials and evaluate electrode performance, Co and Ni doped YCrO3–YSZ composite anodes were developed and tested in H2-containing atmospheres by EIS. The effect of doping on catalytic activity and anode performance was evaluated. The rate-limiting steps and H2 dependence of polarization resistance associated with different dopants were determined. At last a model concerning anode reaction mechanism was proposed based on these results.
Section snippets
Experiment
Y0.8Ca0.2CrO3, Y0.8Ca0.2Cr0.8Co0.2O3, Y0.8Ca0.2Cr0.9Ni0.1O3, denoted as YCC, YCCC and YCCN respectively, were synthesized by EDTA–citric sol–gel method [32], [33]. Standard nitrates (Alfa Aesar) in stoichiometric percentage together with citric acid (Alfa Aesar) were dissolved into distilled water. EDTA powders (Fisher Scientific) as complexant along with ammonia water (Alfa Aesar) was dissolved into the other set of distilled water. The above solutions were blended together followed by
XRD patterns and SEM observation
Fig. 1 shows the XRD spectra from the powder samples of pristine and doped YCCs. All patterns except the one for YCCC calcined at 1100 °C are single phase, showing the orthorhombic perovskite structure (PDF#48-0474). For YCCC treated at 1100 °C, a foreign peak marked with a triangle at around 24.6° was detected and identified as the main peak of Ca2Cr2O5 (PDF#48-0791) by Jade 5.0.
It has been reported by K.J. Yoon, etc. that pure phase Ca and Co co-doped YCrO3 was obtained by glycine–nitrate
Enhancement of performance by doping
The HF arc from 40-μm YCCN sample operated in various in Fig. 8 is associated with the same electrode process as the HF arcs in the 15-μm YCCN sample in Fig. 5, since similar frequency scope, capacitance, and Ea were observed for them. After comparing the f0 and C identified in this study to those reported in literature for SOFC electrodes [46], [47], it is safe to assign the HF arc and LF arc for YCCs in wet 5% H2 to charge transfer and hydrogen adsorption/diffusion processes, respectively.
Conclusions
In this study, doped YCCs as potential anode materials were made and investigated by EIS. The effect of doping on electrode performance was discussed. A possible anode reaction model was proposed based on the results of H2 dependence testing.
It was found that two electrode processes, featured at HF and LF arcs, could dominate the spectra. Charge transfer and surface adsorption/diffusion processes were respectively identified as the corresponding reactions in the HF and LF arcs. The polarization
Acknowledgements
This work is sponsored by US Department of Energy EPSCoR Program under grant number DE-FG02-06ER46299. Dr. Tim Fitzsimmons is the DOE Technical Monitor. Dr. Richard Bajura is the Administrative Manager and Dr. Ismail Celik is the Technical Manager and Principal Investigator of WVU EPSCoR project.
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